The invention relates to semiconductor lasers, and more particularly to broad area semiconductor lasers.
Most high power semiconductor lasers have broad emitter areas, with the result that the lasers typically operate with multiple lateral modes. The generous dimension tolerances of broad area lasers makes them relatively simple to fabricate at lower cost than high power single spatial mode devices.
Narrow stripe devices normally support a single spatial optical mode, and can provide low to moderate optical powers in a high quality beam whose divergence is typically limited by diffraction. Higher output powers are attainable with wider stripes. However, as the stripe width increases, the ability of the stripe to select a single spatial mode is lost and the beam divergence is no longer limited only by diffraction.
Some methods have been suggested for reducing the divergence of the beam produced by wide apertures, including the integration of narrow stripe lasers with flared amplifiers, the incorporation of strong spatial filters, and elaborate schemes of coupled arrays. These solutions all suffer from drawbacks such as astigmatic output, poor efficiency, and high cost.
The figure of merit for broad area lasers is the brightness, which is proportional to the emitted power divided by the product of the aperture width and lateral divergence angle. The product of the aperture width and lateral divergence is referred to as xc3xa9tendue. Thus, brightness is equal to the emitted power divided by the xc3xa9tendue. For a given aperture width, improved brightness can be achieved by increasing the output power and/or by reducing the lateral beam divergence. There is interest in improving the brightness of simple broad area lasers by reducing the lateral divergence angle, especially if this can be accomplished at no extra manufacturing cost.
Present multimode broad area laser sources may be index-guided or gain guided. The index-guided laser generally includes a broad rectangular current injection stripe confined at the stripe ends by two oscillator mirrors, and laterally confined by lower refractive index material. The other type of multimode broad area source is the gain-guided broad area laser, which differs from the index-guided laser in that there is no lateral confinement by lower index material.
Broad area laser sources typically oscillate along those light paths that experience the highest round-trip gain. The index-guided broad area laser generally oscillates in a number of ring modes, since the light path of the ring mode through the gain area is longer, experiencing higher gain than the straight-path modes traveling the shortest path between the mirrors.
The gain-guided broad area laser does not support ring modes and, therefore, oscillates in modes traveling the shortest path between the mirrors. This results in a single-lobed output. However, the counterpropagating standing wave modes produce spatial carrier hole burning and the formation of self focusing filaments. These filaments have little or no phase correlation with each other and, therefore, result in both phase and amplitude variations in the lateral direction. Because the gain guided laser has no built-in spatial mode filter, the lateral beam divergence increases with increasing power.
Therefore, there is a need for a broad area semiconductor laser source that provides for lower lateral divergence operation, and which produces a higher brightness output than is found in current multimode broad area lasers.
Generally, the present invention relates to a broad stripe laser having a folded cavity. The folded cavity permits the laser to produce output powers into a high quality beam with low divergence.
In one particular embodiment of the invention, a laser source has a cavity and includes a first material portion having a first refractive index and a second material portion having a second refractive index which forms forming a first interface with the first material portion. The first interface is disposed within the cavity so that divergence of an intracavity light beam propagating within the first material portion is changed upon reflection from the interface.
In another particular embodiment of the invention, a laser source has a cavity and includes a first material region disposed within the cavity and having a first refractive index and a second material region having a second refractive index less than the first refractive index, and forming a first interface with the first material region. The first interface is disposed to reflect a lowest order reflection mode light beam propagating within the first material region. The first interface is disposed at a predetermined angle so that the lowest order reflection mode intracavity light beam is incident on the first interface at an angle below the critical total internal reflection angle, and higher order reflection modes are incident on the first interface at angles in excess of the total internal reflection critical angle.
In another particular embodiment of the invention, a laser source includes a resonant laser cavity and reflective filtering means, disposed within the laser cavity for selectively filtering a reflective mode within the resonant cavity.
The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments.